This invention relates to solar panels, such as panels for heating water and the like, and, more particularly, to solar panels which have vented air plenums, typically above the collector element, to eliminate overheating.
Solar panels have been used to collect solar energy for space heating, water heating and a variety of other applications requiring heat energy. Solar panels of this type typically have a collector plate or other collector element which is of a color (black or other dark color) and/or texture to maximize absorption of solar radiation. Such collector elements have fluid conduits, often a large number of parallel conduits, disposed in efficient conductive heat-transfer relation thereto. Often the conduits are formed integrally with the collector element. A fluid, usually a liquid, flows (aided normally by a remote pump or fan) through the conduits and is heated by taking heat from the collector element. This heated fluid may be used in a variety of ways, either directly (for example, as domestic hot water) or for a subsequent heat transfer for some other purpose such as space heating. The fluid passing through such conduits may be referred to as the "working fluid".
Often, particularly in cold climates, a glazing, that is, a solar energy-transmitting cover, is placed over the collector element in closely spaced relation thereto. This cover is usually supported by a frame of some sort, and forms an air plenum chamber which isolates and insulates the collector element from the environment to minimize undesirable heat loss. The instant invention is applicable to solar panels of this general type and to any solar panel which has an air plenum adjacent to a collector element. The invention relates to the need to vent such air plenum whenever destructive overheating must be prevented.
Such solar panels may be mounted on a roof or other structure in position to receive solar radiation. Panels of this type are often generally flat and rectangular, and are often mounted at a substantial tilt or angle with respect to the horizontal to be facing the sun as directly as possible. In many cases, several panels may be ganged together to increase collection of solar radiation.
A major problem with covered solar panels of the type described is excessive heating. If the temperature of the solar collector and/or the entire panel rises above certain levels, a great deal of damage can occur. Of course, the temperatures which are tolerable depend on the materials and construction of the panel. Differing thermal expansion coefficients of materials used in the panel may result in the breaking of bonds and joints. And, if the working fluid in the conduits vaporizes, the pressure may increase to the point of rupturing the conduits, thus causing leakage in the system. Very high temperatures for long periods of time may cause permanent distortions in the collector element (particularly metal collector elements) and related parts and can even cause melting of plastics and combustion of combustible materials.
Overheating of a solar panel can arise from several failures in the system, such as a blockage or other failure in the circulation of the working fluid, pump failure, power failure, or malfunctioning of the solar panel control system. Various systems have been made to protect solar panels from overheating. However, many of these systems have failed because of electrical power outages or other problems.
Attempts have been made to develop means of venting the air plenum between the solar collector and the cover when overheating occurs to allow the hot air adjacent to the overheated collector element to escape from the air plenum and be replaced by cooler air from the atmosphere, thus retarding heat buildup and allowing the collector element to cool to acceptable temperature levels. None of the efforts prior to the instant invention, however, have resulted in a practical and effectively operating vented solar panel.
In certain solar panels of the prior art, temperature-responsive bimetal valves or dampers have been placed at opposite ends (the lower and upper ends, considering panel tilt) of the panel. Such arrangements have been unsatisfactory for several reasons. In such panels, the temperature-responsive valves at either end operate separately rather than in a coordinated manner.
The opening of such valves at widely differing times occurs because of the temperature differences between the lower and upper ends of the panel. In many cases, the isolation of the collector element of such a vented panel would be breached without allowing proper venting; such breach, while serving no useful purpose, would allow some unwanted ingress of dirt or dust, particularly near the opened valve. In other cases, venting would stop prematurely because of premature closing of one of the temperature-responsive valves, before sufficient cooling occurs. While the lack of coordination of separately operable venting valves at opposite ends of the solar panel could be avoided by leaving one end of the solar panel open at all times, such would only reduce the efficiency of panel operation and exacerbate the problem of ingress of dust and dirt.
A further problem with some of such temperature-responsive valves is that, even when valves at both ends are open, the venting is insufficient to allow flow of air across substantially the entire width of the solar panel. Instead, flow may occur primarily near the line extending between the bimetal valves at either end of the panel, leaving other portions of the panel substantially uncooled.
Another drawback of the bimetallic valve venting devices is that the bimetallic temperature sensors are at or near the opening, normally an integral part thereof. No flexibility of sensor placement is available.
Another major problem of prior glazed solar panels is their high cost. Glazed solar panels are generally of expensive materials and construction. The collector elements are often of expensive metal materials chosen to avoid corrosion. Some glazed solar panels of the prior art have complicated, powered control systems to prevent problems. Such systems are generally quite expensive, and are also subject to failure because of power outages.
To reduce high panel cost, plastic collector elements could be used. Plastic collector elements, however, undergo much greater dimensional changes than similarly shaped metal elements, and use of such elements would place constraints on panel construction, and on any control systems forming a part thereof.
In summary, there has been need for a relatively low-cost, efficiently operating, and practical vented solar panel. There has been need for a vented solar panel with a reliable heat-sensing device and vent control system.